Two enzymes, a cyclic-AMP-dependent protein kinase and phosphoprotein phosphatase, have been purified to near homogeneity from bovine heart. Meanwhile, histone H1 (or f1), which is phosphorylated by cyclic-AMP-dependent protein kinase at only one site, has been purified from rabbit thymus to serve as an interconvertible substrate. Kinetic experiments are being carried out to quantify the parameters which define a monocyclic cascade system.